3. On the Mappings tab, click Add, and enter aspnet_isapi.dll (the path is under Windows\Microsoft.net\Framework\<version>\...check the existing mapping for .aspx if you need the exact path) as the Executable, and your desired extension under Extension (I used .foo). Clear the "Check that file exists" box. Click OK (if OK is greyed out, tab around in the textboxes a bit, that usually seems to clear the issue that prevents it from being active). Click OK.

4. In your web.config file (or the main web.config for the machine, if you want this to apply to all sites), add the following HttpHandler mapping (inside the <system.web> tags:

5. Also in your web.config, add the following Build Provider mapping (goes between the <compilation> tags, you may need to edit the default tag, since in ASP.NET 2.0 it defaults to a self-closing tag):

6. Add a new web form to the page (probably easiest to stick with single file pages for this), add controls, etc., and when you're finished with the page, rename the extension to the one you configured in IIS.